1. Field of the Invention
This invention relates to batteries, particularly rechargeable batteries that have electronics for protecting the batteries.
2. Description of the Related Art
Rechargeable batteries are used in a variety of electronic devices, including portable computers, portable computer peripherals, personal digital assistants (PDAs), cellular phones, and cameras. Because of the wide variety of uses for rechargeable batteries, a number of different rechargeable battery chemistries have been developed, each having certain advantages and disadvantages. Among the most commonly used battery chemistries are: nickel cadmium (NiCad), nickel-metal hydride (NiMH), lithium ion (Li-ion) and lithium-polymer (Li-polymer).
NiCad batteries have nickel and cadmium electrodes and a potassium hydroxide electrolyte. NiCad batteries are the most common rechargeable batteries, however, NiCad batteries are subject to a number of problems. For example, NiCad batteries have a memory effect, which is a loss of battery capacity caused by recharging the battery before it is fully discharged. Additionally, NiCad batteries are susceptible to over-charging, which causes the battery to develop internal short circuits, thereby causing the battery to run down prematurely which may eventually cause the battery to take no charge at all. Additionally, cadmium is a poisonous heavy metal, and so disposing of NiCad batteries presents additional problems.
NiMH batteries offer higher energy density than NiCad batteries, eliminate many of the disposal problems, and are relatively inexpensive. NiMH batteries have a hydrogen-absorbing alloy anode, a nickel compound cathode, and a potassium hydroxide electrolyte. However, NiMH batteries also have a number of disadvantages. For example, NiMH batteries have a high self-discharge rate, are subject to voltage depression (an effect similar to the memory effect seen in NiCad batteries), and are sensitive to thermal conditions.
In recent years, Li-ion batteries have become the rechargeable battery of choice in devices such as portable computers. The chemistry behind Li-ion batteries involves lithium-plated foil anodes, an organic electrolyte, and lithium compounds within carbon electrodes. Li-ion batteries have very high energy densities, better cycle life than NiMH or NiCad batteries, higher output voltages, and lower self-discharge rates. However, one problem with Li-ion batteries, and potentially other battery chemistries, is the batteries' vulnerability to damage when charged to high capacity (e.g. 85-100% of full capacity) and exposed to high temperatures (e.g. 55.degree. C. or greater). Under these conditions the battery may suffer permanent degradation in performance or even total failure.
Accordingly, it is desirable to have a rechargeable battery pack that protects a rechargeable battery from performance degradation associated with exposure to harmful temperatures when the battery is charged above a particular capacity.